The present invention relates in general terms to devices making it possible to synchronize with one another, the oscillations of two sequences of periodic signals having a high frequency, which is identical or very substantially identical. Such systems have numerous applications, one of the most important being the synchronization of clock signals.
Throughout the remainder of the present text, S will be used for designating pulses emitted by an oscillator, whereof it is wished to make the phase dependent on those of the reference pulses R.sub.f, whose frequency is clearly determined, stable and high.
In general terms, the synchronization of the oscillations of two sequences of periodic pulses of this type has hitherto been carried out with the aid of devices known under the name phase locked loops. Phase locked loops use the principle of phase-frequency servocontrol, whereof briefly the known principle will be described with reference to the attached FIG. 1.
A known phase locked loop essentially comprises an oscillator 2 emitting a sequence of pulses S, a reference oscillator 4 emitting the pulse sequence R.sub.f or reference pulses at a given, stable frequency. The two pulse sequences S and R.sub.f are simultaneously fed into a detector or phase comparator 6, which at the output emits to a dephasing correction system 8 an electrical signal characteristic of the phase deviation possibly measured between the pulses S and R.sub.f. The phase corrector 8, optionally associated with a simple filter, in turn supplies to the oscillator 2 the time correction signal making it possible to displace the frequency of said oscillator 2 to bring it into phase at all times with that of the oscillator 4. When the loop is closed in this way, the oscillator 2 is made dependent in a quasi-perfect manner on the reference oscillator 4 and the sequence of pulses S is available in quasi-perfect synchronism with that of the oscillator 4. Such a known device is in particular described in U.S. Pat. No. 4,121,171.
It is clear that one of the important properties of a servocontrol of this type is its capacity to react very rapidly to the appearance of a phase shift without itself producing instabilities in operation. The stabilities linked with narrow band oscillators and it is desirable, for a high frequency and therefore high speed operation, to have an oscillator with a minimum complexity and which does not have any reactive elements other than the inevitable parasitic capacitances.
In order to produce such a known device according to FIG. 1, two different technologies exist, namely analog constructions and digital constructions. In both cases, the performance characteristics of such a phase locked loop are essentially linked with the characteristics of its internal oscillator 2, which is in fact its main component. On using an analog oscillator, a good frequency stability is obtained due to the crystal oscillators. This stability is associated with the very narrow resonant band of said oscillators, which prevents any possibility of significant frequency variations and therefore rapid resynchronization. Unfortunately in phase locking devices using analog corrections, there is still a significant possibility of interference, which can sometimes lead to a very unstable operation. Moreover, the technological dispersion of said equipments from the manufacturing standpoint can lead to other problems, which are very difficult to control.
Digital oscillators have also been used in phase locked loops. They have the advantage of their stability, their ease of simulation and also simplicity of design. However, their operation is based on a correction principle by variable integral division of a periodic signal from an internal oscillator, so that they are subject to the frequency being exceeded when the oscillating frequency e.g. exceeds 1 megahertz and oscillations occur around the centre frequency, which makes their operation difficult. Thus, in this case the synchronization correction which would have to be provided can be excessive compared with the use frequency.